150 ELEMENTARY FORESTRY. 
person may become sufficiently expert at estimating the heights 
of trees to get along without an instrument. 
The Rate of Growth of a Standing Tree is determined 
by removing from the trunk a small cylinder of wood with a hol- 
low auger, called an accretion borer. On this section of wood 
the annual rings are counted, and their width measured with a 
pocket rule graduated in inches and tenths, or in millimeters. 
Figure 41. The mirror hypsometer in use. 
Where the growth has been slow, and the rings are close, a 
pocket lens may be necessary to enable one to count them. 
When a fuller determination of the rate of growth is desirable, 
trees are felled with an ax, or with a saw, and cut into logs. A 
small saw is easier to carry around, but a longer, heavier saw 
does much faster work. The common logging saw of the Min- 
nesota woods is six feet in length. In making an examination 
of the end of a log the rough graining of the saw must often be 
smoothed away before the rings can be counted readily, and this 
